SMW\Store\Collector eliminate duplicate code and split responsibility

Cache handling was moved into its own SMW\ResultCacheMapper class since
its is only a by-product of a Collector. A Collector is has no direct
responsibility about how results are stored or managed.

SMW\ResultCacheMapper
Code coverage: 100%
CRAP: 13

Change-Id: I40a1548e370e1c4f16c0a1ad6e6621d58334ad5c